A fuel injector is commonly used to pressurize and atomize fuel in an internal combustion engine. In a common hydraulically-actuated fuel injector, a piston and plunger system in a spring cavity transfers hydraulic fluid pressure to the fuel. The piston moves reciprocally up and down within the spring cavity, and the motion of the piston causes the plunger to move, as well. First, fuel is introduced into a fuel cavity beneath the plunger, and hydraulic pressure on the piston forces the plunger down into the fuel cavity to compress the fuel. Since the fuel cavity and plunger are of a smaller cross-section than the spring cavity and piston, the force from the piston through the plunger and to the fuel cavity is magnified accordingly in a known manner for greater efficiency of compression.
Next, one of two things can happen. The plunger can contact, or “bottom out” on, a stop plate at the bottom of the fuel cavity and the plunger is consequently stopped and ready for the next stage of the compression cycle. Often the plunger/stop plate collision can damage one or both components, so this is generally only a secondary method of stopping the plunger. Alternately and usually preferably, the fuel or another fluid present in the fuel or spring cavity becomes pressurized until the fluid's resistance to further compression resists and/or stops the motion of at least one of the plunger and the piston. The latter condition is referred to in the art as a “hydraulic lock”, in which a fluid cannot be compressed any more by the outside pressure placed upon it, and is of primary interest in the below description.
Regardless of the plunger stop mechanism, the compressed fuel is injected into the combustion chamber in a known manner at any suitable point in the plunger motion cycle, thereby vacating the fuel cavity. Finally, a piston spring in the spring cavity forces the piston back up to prepare the fuel injector for the next compression cycle. Hundreds or even thousands of these high-speed and high-stress reciprocal fuel compression cycles occur every minute, which makes efficient and robust operation of the various components of the fuel injector a priority.
Often hydraulic fluid under pressure seeps past the piston and into the spring cavity below the piston during operation of the fuel injector. Since the hydraulic fluid could build up in the spring cavity and hydraulically lock the piston as described above before the fuel is fully pressurized for injection, it is common for a vent hole to be provided at the bottom of the spring cavity to carry any extant hydraulic fluid to a vent line, this evacuation being normally propelled by the downstroke of the piston. This vent hole may also function as an air intake to prevent a vacuum being formed in the spring cavity on the piston upstroke and slowing the motion of the piston.
The stop plate mentioned previously is commonly located at an end of the fuel cavity opposite the plunger. The stop plate acts partially to form the fuel cavity and partially to halt motion of the plunger in a situation when the fuel or hydraulic fluid in the fuel cavity or the spring cavity is insufficient to hydraulically lock the plunger and piston in the preferred manner. Situations that can cause a low fuel situation and subsequent “bottoming out” of the plunger (allowing the plunger to contact the stop plate) include fuel transfer pump failure, air in the fuel supply line, fuel pressure regulator valve failure, the engine's being simply allowed to run out of fuel through neglect or malfunction, and the like. Additionally, while bottoming out is generally not a preferred plunger function, design features and choices with respect to other components may allow the plunger to occasionally bottom out in an otherwise normally functioning fuel injector.
There are two main malfunctions that can result when a plunger bottoms out. The high impact velocity of the plunger on the stop plate can cause material failure and stress damage to one or both components, particularly if repeated contact occurs. Also, and more seriously, the piston spring can overtravel or become overcompressed, either of these causing a permanent reduction in the height of the piston spring or even breakage of that spring. Since the piston spring is the only force outside the hydraulic lock acting to resist downward motion of the plunger, a shortened piston spring will probably allow the plunger to bottom out repeatedly until the fuel injector totally fails because of component breakage. It is estimated that this total injector failure occurs within about twenty seconds of the piston spring failure, leaving little to no time for the problem to be detected and the engine shut down to prevent such failure. When the fuel injector fails, the engine effectively loses power in that cylinder and numerous well-known problems typically result.
Additionally, there are many other applications in the field for a piston assembly such as that described above. Any hydraulic piston assembly working to compress a fluid in much the same manner, perhaps in an injection molding or glue-applying situation, would be subject to these or similar difficulties. Since the overall structure of these piston assemblies is analogous to the fuel injector described, it is intuitively obvious that many different applications can be effected by piston assembly failure as described. Therefore, a solution to the piston assembly failure is widely sought.
The present invention is directed to overcoming one or more of the problems as set forth above.